The president of Ethiopia's Amhara region and his top adviser were killed in an attempted coup in which the country's army chief was also shot dead, the office of Ethiopian Prime Minister Abiy Ahmed said.

Spokeswoman Billene Seyoum told journalists a "hit squad" led by Amhara's security chief Asaminew Tsige burst into a meeting in the state offices of Amhara's capital, Bahir Dar, on Saturday and shot regional government President Ambachew Mekonnen and his adviser Ezez Wassie.

The men were "gravely injured in the attack and later died of their wounds," she said.

"Several hours later, in what seems like a coordinated attack, the chief of the staff of the national security forces Seare Mekonnen was killed in his home by his bodyguard in Addis Ababa."

Also shot dead was a retired general who had been visiting him, Billene added.

The bodyguard has been apprehended while Asaminew is still on the loose, sources said.

Al Jazeera's Leah Harding, reporting from Addis Ababa, said Abiy called those responsible "mercenaries".

"The army intelligence general said the coup was meant to create chaos and division in the military. He said the military now has control over the situation ... and he reiterated that there are no divisions within the military," Harding reported.

"This is particularly important because the two generals that were killed in Addis Ababa are part of the Tigre ethnic group, and the person who we believe is responsible for the coup plot is part of the Amhara group."

Analysts said the incident showed the seriousness of the political crisis in Ethiopia, where efforts by Abiy to loosen the iron-fisted grip of his predecessors and push through reforms have unleashed a wave of unrest.

"These tragic incidents, unfortunately, demonstrate the depth of Ethiopia's political crisis," said International Crisis Group analyst William Davison.

"It is now critical that actors across the country do not worsen the instability by reacting violently or trying to exploit this unfolding situation for their own political ends," the expert said. 

Residents of Bahir Dar said late on Saturday there was gunfire in some neighbourhoods and some roads had been closed off.

The US embassy issued alerts about reported gunfire in Addis Ababa and violence around Bahir Dar.

Early on Sunday, Brigadier General Tefera Mamo, the head of special forces in Amhara, told state television that "most of the people who attempted the coup have been arrested, although there are a few still at large."

Since coming to power last year, Abiy has tried to spearhead political reforms to open up the once isolated, security-obsessed Horn of Africa country of 100 million people.

He has released political prisoners, lifted bans on political parties and prosecuted officials accused of gross human rights abuses, but his government is battling mounting violence.

Ethnic bloodshed - long held in check by the state's iron grip - has flared up in many areas, including Amhara, where the regional government was led by Ambachew Mekonnen.

"Since Abiy Ahmed took power and the country moved towards democratisation ... there have been different forms of mobilisations, by different actors, particularly nationalists." Awol Allo, a lecturer in law at Keele University, told Al Jazeera.

"[In] Amhara regional state, there is this feeling that they were marginalised, and these individuals that were suspected to be behind the coup recently said that Amhara people have never been subordinated.. so there is this sense of grievance and victimhood that is driving the nationalist movements," he added.

Ethiopia is due to hold a national parliamentary election next year. Several opposition groups have called for the polls to be held on time despite the unrest and displacement.

Al Jazeera